This invention relates to data communication systems, and more particularly to an RF packet communication system in which a number of remote units send data to a central computer via intermediate base stations.
Bar code readers used in retail or commercial facilities have been, in previous systems, usually connected to a central computer by physical wiring. This connection is quite suitable for permanently-mounted bar code readers as used in supermarket checkout counters, or for hand-held scanners or wands used at similar fixed locations. However, when the bar code reader is to be used by a person who is moving about a building, or when temporary installations are employed, physical wiring is unsuitable, or is at least quite inconvenient. A radio frequency (RF) link can be used to send data from the a hand-held bar code scanner to a central station, or to a local relay point, but the RF links that have previously been available for this purpose have had characteristics making them expensive and inconvenient. These RF links typically have used RF bands requiring F.C.C. licensing for each installation, adding to the cost and administrative burden. The RF components employed in this type of equipment have to be of high precision so that frequency drift and bandwidth spillage are kept within F.C.C. tolerances. In addition, battery drain has required either large, heavy batteries, or frequent recharging, or both. The RF transmission methods previously used also have limited the number of portable terminals which could be used in a given area due to use of a relatively narrow bandwidth. Examples of bar code readers using local RF data links include portable terminals commercially available from the following companies: MSI Data Corporation, Vectran Corporation, LXE Corporation, Norand Corporation, and Telxon Corporation. Portable bar code readers having long-distance RF links are available from Mobil Data International and from Motorola, Inc. (the KDX1000).
Indoor RF communications networks of the voice type have been proposed, such as that of U.S. Pat. No. 4,789,983 for "Wireless Network for Wideband Indoor Communications", or U.S. Pat. No. 4,639,914 for "Wireless PBX/LAN System".
The remote terminals in these prior systems are addressable at any time, i.e., always activated, so the requirements for power are dictated by this feature. In addition, these prior systems have used RF frequency bands requiring F.C.C. licensing of individual users. For these reasons, prior systems of this type have been too costly and otherwise unsuitable for the present purposes.
Spread spectrum wireless transmission is able to use a band that is designated as an "unlicensed" band by the F.C.C. and so licensing is not a factor, and the use of spread spectrum techniques allows the transmission to be accomplished in a reliable manner even though this band is subject to interference from the many diverse users. In U.S. Pat. No. 4,672,658 for "Spread Spectrum Wireless PBX", a system is shown in which each separate user transceiver is matched with a separate transceiver at the central PBX, and each one of these matched pairs transmits with a unique direct sequence spread spectrum chipping pattern. A separate call set-up transceiver having a common direct sequence chipping pattern is used for exchanging information involved in setting up a call. As above, this system requires continuous monitoring of the RF bands by all of the transceivers, and is a voice oriented system requiring varying time periods of maintaining connections, as well as requiring connection from user to user, rather than user to central station. Another example of use of spread spectrum in a local RF link is a utility meter reading system wherein a utility truck driving by a house activates a reader by a CW transmission then receives the data from the reader.
Wireless data communications between a central computer and several remote terminals located within a building, using direct-sequence spread-spectrum techniques to overcome multipath interference, is described by Freret et al, NTC Record, November., 1980, but again these types of systems rely upon continuous operation of the portable units, and impose burdens on the RF circuitry in the remote units which result in complex and expensive construction.
In U.S. Pat. No. 4,740,792 a data transmission system using spread spectrum RF is illustrated wherein vehicles are provided with a transmitter, but no receiver, and the location of each vehicle is reported to a central station periodically by a transmitted packet. The transmitter is powered up only for a very limited duty cycle, so battery drain is minimized. This system has no ability to send data from a central station to one of the vehicles, or to allow the transmitter at the vehicle to receive an acknowledge signal indicating receipt of the data transmitted.
In prior application Ser. No. 374,452, filed Jun. 29, 1989, now U.S. Pat. No. 5,029,183, a packet data communication system is disclosed using a message-exchange protocol in which a base station only sends a message to a remote terminal in response to a message from the remote terminal. The remote terminal sends a message to the base station only at a time of its own choosing, so the transceiver of the remote terminal need not be continuously powered. The base station responds to a message from a remote terminal within a rigid time window following the message from the remote terminal. If there is a need to send a message originating at the base station (or at a central computer), then the base station must wait until a message is received from the remote before it can send the waiting message as part of its response in the fixed time window. The system of U.S. Pat. No. 5,029,183, in an example embodiment, employs spread spectrum RF transmission.
The system of U.S. Pat. No. 5,029,183 provides a great improvement in the power drain on a battery in a remote terminal, and thus allows construction of smaller, lighter, and cheaper remote terminals. To improve the communication channel from the base or central computer to the remote units, a way of periodically notifying the remote terminals that messages are waiting to be sent to them would reduce the average delay time between message initiation and message receipt. As disclosed in U.S. Pat. No. 5,029,183, this communication channel may be facilitated by having the remote units periodically (every half-minute or so) send a message to their base station, even though there is no new data to send, so that if a message to the remote is waiting it can be sent with the reply in the fixed window. This method provides two-way communication when only one side can originate messages, but it nevertheless requires the remote units to periodically energize their transmitters, and it adds to traffic on the RF medium.
It is an object of the present invention to provide an improved, low-cost, low-power, packet data communication network in which a number of remote terminal units are in a form of two-way communication with a central station, preferably a network using an RF link or the like so that the remote units may move about freely in an area to be covered by the network. Another object is to provide an improved packet transmission network in which remote terminal units may be of low cost, low power and small size, yet provide reliable and fast response, as may be needed in a commercial facility (usually indoor) using bar code scanners or the like for data gathering. Another object is to provide an improved protocol for use in a packet data transmission network which results in reliable and high-performance operation, low power consumption and low cost implementation. A particular object is to provide an RF data link for portable terminals usable without site licensing under F.C.C. regulations, so that the expense and delays incident to such licensing are eliminated or minimized. Another object is to provide a method of reducing the delay between message initiation at a base terminal and message receipt at a remote terminal in a system in which only the remote terminals can originate message transmission.